Chapter 31 | Radioactivity and Nuclear Physics
normal amount of 14 C . Estimate the minimum age of the
57. Natural uranium is 0.7200% 235 U and 99.27% 238 U .
2 10 = 1024 .
What were the percentages of 235 U and 238 U in natural
charcoal, noting that
45. A 60 Co source is labeled 4.00 mCi, but its present
uranium when Earth formed
7
activity is found to be 1.85Ã10 Bq. (a) What is the present
activity in mCi? (b) How long ago did it actually have a
4.00-mCi activity?
58. The
46. (a) Calculate the activity
R in curies of 1.00 g of
226
Ra .
(b) Discuss why your answer is not exactly 1.00 Ci, given that
the curie was originally supposed to be exactly the activity of
a gram of radium.
4.5Ã10 9 years ago?
Î² â particles emitted in the decay of
3
H (tritium)
interact with matter to create light in a glow-in-the-dark exit
sign. At the time of manufacture, such a sign contains 15.0 Ci
3
of H . (a) What is the mass of the tritium? (b) What is its
activity 5.00 y after manufacture?
found in living tissue is 0.250 Bq.
59. World War II aircraft had instruments with glowing radiumpainted dials (see Figure 31.2). The activity of one such
5
instrument was 1.0Ã10 Bq when new. (a) What mass of
226
Ra was present? (b) After some years, the phosphors on
48. Mantles for gas lanterns contain thorium, because it forms
an oxide that can survive being heated to incandescence for
long periods of time. Natural thorium is almost 100% 232 Th ,
the dials deteriorated chemically, but the radium did not
escape. What is the activity of this instrument 57.0 years after
it was made?
47. Show that the activity of the 14 C in 1.00 g of 12 C
with a half-life of
1.405Ã10 10 y . If an average lantern
60. (a) The 210 Po source used in a physics laboratory is
1.0 ÂµCi on the date it was
mantle contains 300 mg of thorium, what is its activity?
labeled as having an activity of
49. Cowâs milk produced near nuclear reactors can be tested
for as little as 1.00 pCi of 131 I per liter, to check for possible
prepared. A student measures the radioactivity of this source
with a Geiger counter and observes 1500 counts per minute.
She notices that the source was prepared 120 days before
her lab. What fraction of the decays is she observing with her
apparatus? (b) Identify some of the reasons that only a
fraction of the Î± s emitted are observed by the detector.
reactor leakage. What mass of 131 I has this activity?
50. (a) Natural potassium contains 40 K , which has a halfhave a decay rate of 4140 Bq? (b) What is the fraction of
40
K in natural potassium, given that the person has 140 g
61. Armor-piercing shells with depleted uranium cores are
fired by aircraft at tanks. (The high density of the uranium
makes them effective.) The uranium is called depleted
because it has had its 235 U removed for reactor use and is
in his body? (These numbers are typical for a 70-kg adult.)
nearly pure 238 U . Depleted uranium has been erroneously
51. There is more than one isotope of natural uranium. If a
researcher isolates 1.00 mg of the relatively scarce 235 U
called non-radioactive. To demonstrate that this is wrong: (a)
Calculate the activity of 60.0 g of pure 238 U . (b) Calculate
and finds this mass to have an activity of 80.0 Bq, what is its
half-life in years?
the activity of 60.0 g of natural uranium, neglecting the 234 U
life of
1.277Ã10 9 y. What mass of
40
K in a person would
52. 50 V has one of the longest known radioactive half-lives.
In a difficult experiment, a researcher found that the activity of
1.00 kg of 50 V is 1.75 Bq. What is the half-life in years?
53. You can sometimes find deep red crystal vases in antique
stores, called uranium glass because their color was
produced by doping the glass with uranium. Look up the
natural isotopes of uranium and their half-lives, and calculate
the activity of such a vase assuming it has 2.00 g of uranium
in it. Neglect the activity of any daughter nuclides.
54. A tree falls in a forest. How many years must pass before
the 14 C activity in 1.00 g of the treeâs carbon drops to 1.00
decay per hour?
55. What fraction of the 40 K that was on Earth when it
formed
4.5Ã10 9 years ago is left today?
56. A 5000-Ci 60 Co source used for cancer therapy is
considered too weak to be useful when its activity falls to
3500 Ci. How long after its manufacture does this happen?
and all daughter nuclides.
62. The ceramic glaze on a red-orange Fiestaware plate is
U 2 O 3 and contains 50.0 grams of 238 U , but very little
235
U . (a) What is the activity of the plate? (b) Calculate the
total energy that will be released by the
238
U decay. (c) If
energy is worth 12.0 cents per kW â h , what is the monetary
value of the energy emitted? (These plates went out of
production some 30 years ago, but are still available as
collectibles.)
63. Large amounts of depleted uranium (
238
U ) are
available as a by-product of uranium processing for reactor
fuel and weapons. Uranium is very dense and makes good
counter weights for aircraft. Suppose you have a 4000-kg
238
block of
U . (a) Find its activity. (b) How many calories
per day are generated by thermalization of the decay energy?
(c) Do you think you could detect this as heat? Explain.
64. The Galileo space probe was launched on its long journey
past several planets in 1989, with an ultimate goal of Jupiter.
238
Its power source is 11.0 kg of
Pu , a by-product of
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